Heating device

ABSTRACT

Heating devices are typically used in glue guns to soften adhesives. A conventional glue gun employs a resistive element to generate heat energy. The invention provides a glue gun comprising an amplifier ( 2 ) in communication with a capacitor ( 7 ). The amplifier is arranged to supply a.c. power at a predetermined frequency to plates ( 8, 9 ), of the capacitor. Adhesive ( 10 ) is introduced between the capacitor plates an is softened by dielectric heating.

This invention relates to heating devices and to methods of using suchdevices to heat, for example, adhesive materials.

Certain adhesive materials need to be softened by heating for use. Thetwo main types of such adhesives are thermoplastic adhesives andthermosetting adhesives. Thermoplastic adhesives can be softened anynumber of times by heating. Upon cooling, they once again adhere.Thermosetting adhesives undergo an irreversible chemical change whenthey harden. After setting, these adhesives retain their strength and donot soften when heated.

Conventionally, a so-called glue gun is used to soften and apply suchadhesives. A typical glue gun employs a resistive element used toconvert electrical energy to heat energy, to heat the adhesive.

The invention provides heating apparatus comprising an amplifier incommunication with a capacitor, the amplifier being arranged, in use, tosupply power to plates of the capacitor, between which plates asubstance to be heated may be introduced.

The invention has particular application in the softening by heating ofadhesives. Accordingly, the invention further provides a glue guncomprising an amplifier in communication with a capacitor, the amplifierbeing arranged, in use, to supply power to plates of the capacitor,between which plates an adhesive to be softened may be introduced.

Advantageously, the amplifier includes an active device, such as atransistor, arranged to perform substantially as a switch.

Preferably, the active device is driven so that the amplifier producesa.c. power at a predetermined frequency. A Class E type amplifier isparticularly suitable for putting the invention into effect.

Preferably, the capacitor is part of a resonance circuit, i.e. a circuitincorporating an appropriate inductor.

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram for apparatus constructedaccording to the invention; and

FIG. 2 illustrates a glue gun employing the circuit of FIG. 1.

With reference to FIG. 1, a circuit constructed according to theinvention is shown and indicated generally by the reference numeral 1.The circuit 1 comprises two main parts: a power amplifier 2 and a load 3for the amplifier.

The power amplifier 2 comprises a driver 4, which controls an activedevice. In this embodiment, the active device is in the form of a powertransistor 5. The transistor 5 acts substantially as a switch whenappropriately driven by the driver 4. Thus dc energy from a power supply(not shown) is converted into ac energy at the switching frequency. Theoutput ac power is coupled to the load 3.

The load 3 comprises a coil 6 and a capacitor 7, which together form aseries resonance circuit The capacitor 7 has plates 8, 9, between whicha sample 10 of a substance to be heated is introduced. The sample 10thus forms the dielectric of the capacitor 7. The capacitance of thecapacitor 7, and hence the resonant frequency of the load 3, isdependent on the dielectric constant of the substance between the plates8,9. Thus, the choice of substance to be heated influences the frequencythat the power amplifier 2 has to produce in order to generate resonancein the load 3.

In use the power amplifier 2 delivers power, switched at a highfrequency, to the load 3. Thus, the sample 10 is subjected to ahigh-frequency alternating current field that produces heat in thesample resulting from molecular friction. In this manner, the sample 10is heated by the apparatus of the invention.

The circuitry of FIG. 1 is particularly suitable for use in softeningadhesives, and so may be usefully incorporated in a glue gun, such as isshown in FIG. 2.

The glue gun, indicated generally by the reference numeral 11,incorporates the circuit 1. The gun also includes a trigger 12 fordrawing adhesive 13 in stick form into the gun. There are also guides(not shown) inside the gun for guiding the adhesive stick 13 into placebetween the plates 8,9 of the capacitor 7. In use, the adhesive issoftened by dielectric heating and is then drawn from a nozzle 14 forapplication to parts to be adhered.

Preferably, the amplifier is of the Class E type.

The power amplifier is arranged to deliver power at a predeterminedfrequency, which frequency produces dielectric heating of the substance,e.g. adhesive, between the capacitor plates. Thus, if a user insertsbetween the plates a substance having a different dielectric constant,no dielectric heating occurs owing to the different resonant frequency.This is an important safety feature.

The invention is not limited to heating adhesives, and may be applied todrying, cooking and other similar processes involving dielectricheating.

1. Heating apparatus comprising an amplifier in communication with acapacitor, the amplifier being arranged, in use, to supply power toplates of the capacitor, between which plates a substance to be heatedmay be introduced.
 2. Apparatus as claimed in claim 1, in which theamplifier includes an active device arranged to perform substantially asa switch.
 3. Apparatus as claimed in claim 2, in which the active deviceis a transistor.
 4. Apparatus as claimed in claim 2, in which the activedevice is driven so that the amplifier produces a.c. power at apredetermined frequency.
 5. Apparatus as claimed in claim 1, in whichthe amplifier is of the Class E type.
 6. Apparatus as claimed in claim1, in which the capacitor is part of a resonance circuit.
 7. (Cancelled)8. A glue gun including heating apparatus as claimed in claim 1, thesubstance being an adhesive to be softened by heating.
 9. A glue gun asclaimed in claim 8, further comprising means for introducing adhesivebetween the capacitor plates.
 10. A glue gun as claimed in claim 9, inwhich the means for introducing adhesive includes a trigger.
 11. A gluegun as claimed in claim 8, further comprising a nozzle arranged todeliver softened adhesive for use.
 12. (Cancelled)
 13. A method ofheating a substance, comprising the steps of introducing the substancebetween the plates of a capacitor and supplying a.c. power to thecapacitor by means of an amplifier.
 14. A method of softening anadhesive, comprising the steps of introducing the adhesive between theplates of a capacitor and supplying a.c. power to the capacitor by meansof an amplifier.